This invention relates to a laminated dielectric resonator and a dielectric filter which are chiefly used in high-frequency radio tools such as a portable phone. The laminated dielectric resonator is solely used as a resonant element such as a high-frequency oscillation circuit, or used, as combination of a plurality of laminated dielectric resonators, for composing a dielectric filter working as a band-pass filter or a band elimination filter.
Accompanied by development of vehicular communication, small-sized portable phones have been desired. Size reduction of parts to be used therein is the key for reducing the size of high-frequency radio tool such as a portable phone. Since a dielectric filter widely used as a high-frequency filter is one of high-frequency parts which largely occupies the radio circuit of the portable phone, the size reduction thereof is desired.
The dielectric filter is composed of a plurality of dielectric resonators which are cascade-connected to one another via joint elements. Conventionally, a coaxial dielectric resonator in which an electrode is formed on a surface of coaxial ceramic element is used for the dielectric resonator, and the conventional dielectric filter is composed of the coaxial dielectric resonators. However, since micro-fabrication of the ceramic in manufacturing the coaxial dielectric resonator is too limited to be thinned, a laminated dielectric resonator which is composed of a plane-type strip line resonator is contemplated.
One example of the conventional laminated dielectric resonators is explained, with reference to drawings. FIG. 15(a) is a perspective exploded view of the conventional laminated dielectric resonator. FIG. 15(b) is a section, taken along a line X--X' in FIG. 15(a).
In FIGS. 15(a), (b), a strip line 36 is formed on a first dielectric sheet 35, and shield electrodes 7 are respectively provided on and under the strip line 36 via dielectric sheets 35, 37 laminated thereon and thereunder. One end of the strip line 36 is grounded via a ground electrode 9 so as to compose an end-short strip line resonator. Impedance at an open end is infinite with a frequency corresponding to a wavelength of electromagnetic wave which is as four times as the length of the strip line 36, so as to perform parallel resonance. Such a laminated dielectric resonator is disclosed, for example, in FIG. 1 of Laid Open unexamined Japanese Patent Application No.2-290303.
Under the above construction, however, the resonator can be thinned but has conventional length. The dielectric ceramic material to be laminated is so limited that the dielectric material is limited to low-permittivity material, with a result of longer resonator than the conventional one. In order to reduce the whole length of the resonator, a relative permittivity of the dielectric material must be high because the resonant frequency depends on propagation wavelength on the strip line. However, the dielectric material with high relative permittivity is generally burnt with too high temperature to burn with an electrode (hereinafter referred to it as internal electrode) arranged in the dielectric material, which restrains the size reduction. Further, the dielectric material with high relative permittivity generally has a large dielectric loss tangent which lowers unloaded Q of the laminated dielectric resonator, with inferior temperature characteristic with respect to frequency. As a result, the characteristic of the laminated dielectric resonator is degraded.
The above-mentioned Japanese reference proposes that a strip line is formed on each of two dielectric sheets laminated, and the strip lines are connected to each other to be formed in two-fold configuration. However, while reducing the physical length of the resonator by the two-fold configuration, further reduction thereof is difficult.
FIG. 16 is a perspective exploded view of an antenna duplexer composed of a conventional dielectric filter. The antenna duplexer is so composed that two filters of a transmission filter and a receiving filter are combined. The prior art dielectric filter is explained below, referring to the antenna duplexer in the figure as an example. In FIG. 16, reference numerals 701-706 denote coaxial dielectric resonators, 707 denotes a coupling substrate, 708 denotes a metallic case, 709 denotes a metallic cover, 710-712 denote series capacitors, 713 and 714 denote inductors, 715-718 denote coupling capacitors, 721-726 denote connection pins, 731 denotes a transmission terminal, 732 denotes an antenna terminal, 733 denotes a receiving terminal, and 741-747 denote electrode patterns formed on the coupling substrate 707.
The coaxial dielectric resonators 701, 702, 703, the series capacitors 710, 711, 712 and the inductors 713, 714 compose a transmission band elimination filter. The coaxial dielectric resonators 704, 705, 706 and the coupling capacitors 715, 716, 717, 718 compose a receiving band pass filter.
The transmission filter is connected at one end thereof to the transmission terminal 731 to be electrically connected to a transmitter, and is connected at the other end thereof to one end of the receiving filter and to the antenna terminal 732 to be electrically connected to an antenna. The other end of the receiving filter is connected to the receiving terminal 733 to be electrically connected to a receiver. The antenna duplexer composed of the conventional dielectric filter under such a construction is disclosed, for example, in FIG. 4 of "RF Front End Circuit Components Miniaturized Using Dielectric Resonators For Cellular Portable Telephones" by T. Nishikawa, IEICE Transactions, Vol.E74, No.6, pp.1556-1562, June, 1991.
However, such a construction requires a number of electronic parts such as capacitors and inductors or mechanical parts such as connection pins, which involves a problem that reduction of size and cost is difficult.